The present invention relates to a method and system of drilling for deepwater oil and gas reserves. More particularly, the present invention relates to a riser system through which offshore drilling operations are conducted from a surface vessel or platform.
Drilling for oil and gas counterbalances normal geopressure with a hydrostatic head of weighted drilling fluid commonly referred to as "mud." However, drilling operations sometimes encounter rapid increases in pressure known as "kicks." Blowout preventers ("BOPs") are used to contain such pressure increases during drilling and other well operations and have been adapted for offshore application. Deepwater drilling has traditionally deployed such BOPs either at the wellhead far below on the ocean floor or at the surface and connected to the wellhead through a single string high pressure riser. Both of these conventional approaches have benefits. However, each approach provides its benefits only at the cost of accepting substantial detriments.
For example, a single string high pressure riser with a surface BOP facilitates simpler well operations, riser handling and BOP maintenance. However, the pressure requirements lead to a very thick wall, heavy and expensive riser system. The high weight of the riser also tends to increase the tension required to hold up the riser in deepwater and accommodating this extra tension can seriously and adversely affect platform costs. Further, such risers are adversely impacted by riser cleaning and wear problems in deepwater deployment.
Alternatively, use of subsea BOP stacks provide high pressure shut off at the mudline, decreases the tension requirements and benefits from well established procedures. However, the heavy subsea BOP stack and associated equipment are difficult to handle, maintain and store. Further, accommodating subsea BOP stacks at the wellhead requires increased well spacing at the sea floor, which, for vertical access, requires an increase in the size of the wellbay of the surface facilities. This, in turn, adversely affects overall platform costs. Further, accommodating such storage, handling and dimensional challenges can lead to a dedicated, purpose built rig for drilling and well operations for which a modular, temporarily deployed rig would otherwise prove satisfactory.
There is thus clearly a need for a riser system and drilling method for deepwater hydrocarbon developments that provides the benefits of a surface BOP without the difficulties associated with a conventional high pressure riser.